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AnoverviewonGemopals:fromthegeologytothecolortothemicrostructure

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10 Thirteenth Annual Sinkankas Symposium—Opal

Overview of Gem Opals: From the Geology to Color and Microstructure 11

Opal is one of the most fascinating gems, es-pecially in the precious form that displaysthe spectacular rainbow f lashes. Its appear-

ance constantly changes when one looks at it, and wecannot really qualify its colors and grasp its essence,giving opal a unique, mysterious aspect.

Opal has a nomenclature of its own, dependingon who is examining it: the mineralogist, the geol-ogist, the gemologist, or the jeweler. We hear termssuch as hyalite, jelly, noble, precious, fire, harlequin, blackopal, and many more. These qualifiers can becomeconfusing, but we will see that most of them referto the opal’s transparency, its body color, or thepresence (or absence) of the rainbow f lashes re-ferred to as play-of-color. To better understand allof these features, it is necessary to explore opal’sformation mode and its internal structure, subjectsthat will be developed here, and to examine the rea-sons for the many body colors opals can display.

What is an Opal?Opal is a mineral composed of silica and water, de-fined by the formula SiO2

, nH2O, with n represent-ing the variable amount of water from one specimento another. The water content usually varies from4% to 10% (e.g., Bayliss and Males, 1965), but withextreme values from 0.8% to 21% (e.g., Sosnowskaet al., 1997; Webster, 1975). An opal is not crys-

talline, but either amorphous or microcrystalline.In other words, it will not develop crystal faces andits atomic arrangement is poor or nonexistent.

ClassificationBased on the definition given above, mineralogistsclassify two kinds of opals: (1) amorphous opal, re-ferred to as opal-A; (2) microcrystalline opal, re-ferred to as opal-CT, which stands for disorderedα-cristobalite with significant α-tridymitic stacking.Cristobalite and tridymite are quartz polymorphs—that is, of the same chemical composition, SiO2 (sil-ica), but with different crystal structure. The amountof disorder may vary between the cristobalitic andtridymitic end-members (e.g., Ilieava et al., 2007).

Geologists define by the rock in which it is found:sedimentary opal or volcanic opals. Opal is a second-ary mineral in both, meaning it was formed after therock itself, by a weathering process of a silica-richrock, and f luid re-precipitating in empty spaces of thehost rock. We will return to this concept later on. Thetypical example of a sedimentary opal deposit is Aus-tralia, where opal forms in sandstone of the GreatArtesian Basin. Australia is still the leading producerof gem-quality opals around the world. Typical vol-canic opals are found in Mexico and in the more re-cent Ethiopian deposits, for example (Figure 1). Thevolcanic rocks are rhyolitic in composition, a silica-rich volcanic rock.

Figure 1:Nodules of opal in a rhyolitic rock from Mexico. Opal formed in available space of the silicic volcanic rock, which is inthis case was in the vugs of the rhyolite. Note that there are several kinds of opal nodules: a transparent opal with play-of-col-ors; a common fire opal, a yellow opal, and on the left, an opaque red opal. Specimen from MINES Paristech, no. 16209, 17 x 12cm. Photo by Eloïse Gaillou.

An Overview of Gem Opals: From the Geology to Color and Microstructure

Eloïse Gaillou

12 Thirteenth Annual Sinkankas Symposium—Opal

Gemologists and jewelers first defined opal bywhether it displayed play-of-color (rainbowf lashes). Opal that does show play-of-color is re-ferred to as “precious” or “noble.” Otherwise it isreferred to as a “common” opal. Yet the term “com-mon” is not glorifying, so we usually call attentionto its other attractive aspects, such as transparencyand body color. “Jelly” opal, for example, is a trans-parent (and most often colorless) opal. “Girasol”opal refers to a translucent material with a milky as-pect. There are “fire” and “chocolate” and countlessother color qualifiers.

Note that the qualifiers used to define color andtransparency may also be used for precious opals.For example, there are some common fire opal(called by usage “fire opal”) and some play-of-colorfire opal (called “precious fire opal,” Figure 2).Maybe the most confusing term is “black opal,” foropal from Australia. It refers to an opal that has adark body color (usually dark gray) and exhibitsplay-of-color. Some qualifiers also indicate a patternof the play-of-color, such as the term “harlequin”for a mosaic-like pattern of play-of-colors (whichby definition seems to require a red component).The “digit” pattern (Figure 3) is a finger-like appear-ance sometimes seen in opals from Ethiopia and Ne-vada: The play-of-color is restricted to columnarareas, in between a cement of common opal (Ron-deau et al., 2010, 2013).

FormationAs mentioned previously, opal is a secondary min-eral formed by alteration at low temperature of a sil-ica-rich host rock. By chemical analyses, Gaillou etal. (2008a) showed that opals and their host rockcontain the same chemical elements, but in muchlower quantity in the opal, which incorporates mostof them as impurities (less than 1%). Opal’s chemicalsignature is close to a dilution of the host rock (Fig-ure 4). This is indicative of a weathering process ata small scale, and precipitation in situ of a silica-richf luid in empty spaces of the host rock.

There are other ways opal can form, such asthrough biomineral processes, but those do not pro-duce gem opals (only very porous white chalkyopals) and therefore will not be examined here. Theprocesses controlling the formation of play-of-colorversus common opals are not completely under-stood, nor are the processes that control precipita-tion versus crystallization of other form of silica.

Body ColorThe body color of a gem represents the intrinsiccolor of the mineral, such as red for a ruby, blue fora sapphire, or green for an emerald. Opal comes ina variety of colors that span the entire visible spec-trum: from violet, blue, green, yellow, orange, andpink to red. It comes also in colorless, milky, white,brown, gray, and black. As for all poorly crystallizedsilica, colors come from the presence of inclusionscolored by transition metal ions that absorb part ofthe visible spectrum of light. The reader is referredto the articles of Fritsch et al. (1999), Gaillou et al.(2008a), and Rondeau et al. (2013). Note that the fol-lowing list is not exhaustive, but gives a goodoverview of the main opals found in the market.

Typically, opals ranging from yellow to orange tored to brown have increasing content of Fe, up toabout 6500 ppm in some dark brown opals fromMezezo in Ethiopia. The origin of their color is dueto nanometric inclusions of an iron-rich mineral,

Overview of Gem Opals: From the Geology to Color and Microstructure 13

Figure 4: Rare-earth elements analyzed in a Mexican opaland its host rock, normalized to the composition of chon-drite (from Gaillou et al., 2008a). The opal contains aboutone-tenth the concentration of rare-earth elements as itshost rock. Both have similar patterns, though. This is typi-cal of a weathering process.

Figure 2 (facing page, top): This play-of-color fire opal,also referred to as precious fire opal, measures 6 mmacross. Photo by Eloïse Gaillou.

Figure 3 (facing page, bottom): A play-of-color opal fromMezezo, Ethiopia, showing the digit pattern. The opalmeasures approximately 6 cm. Photo by FrancescoMazzero/Opalinda.

such as hematite. Opaque yellow and orange gemmyopals also exist, and they come from Saint-Nectairein France and from Austria (called “forcherite”).Their color is due to nanometric inclusions of real-gar and pararealgar (not orpiment, as it was longstated). Pink opals from Mexico (Figure 5), Peru, andFrance are colored by quinones that are adsorbed onsome phyllosilicates (palygorskite for Mexican andPeruvian opals, sepiolite for the French opals calledquincyite). Pink opals from Montana are different,as their coloring agent is cinnabar in micrometer in-clusions. Purple opals from Madagascar contain f lu-orite that gives rise to the overall purple colorzoning. The attractive aqua-blue color of Peruvianand French (from Biot) opals is due to inclusionsrich in copper (Cu) and magnesium (Mg), possiblybisbeeite. The sky blue color of some opals from Ari-zona, Oregon, and Madagascar (and possibly fromBrazil) is due to a phenomenon of diffraction oflight on some nanometric inclusions of cristobalite.Typical of the diffusion phenomenon, sky-blue opalslook orangy when a strong light is put behind them.Green opals referred to as chrysopals in the markethave undefined nickel-rich inclusions.

LuminescenceThe luminescence of opals, like their body color, ismostly constrained by the presence of impurities. Itis mainly controlled by two elements: Fe (iron) andU (uranium). Fe will be the quencher of lumines-cence, and more specifically Fe3+. The diagram inFigure 6 shows that none of the samples examinedby Gaillou et al. (2008a) containing more than about

14 Thirteenth Annual Sinkankas Symposium—Opal

Figure 6:Opal’s luminescence depends on the concentra-tion of uranium and iron.

Figure 5:This common pinkopal from Jalisco, Mexico, is1.2 cm wide.

3000 ppm of Fe showed any luminescence. Whenthe opals contained more than 1 ppm of U and lessthan 3000 ppm of Fe, they exhibited a green lumi-nescence (Figure 7, top). UO2

2+, the uranyl group, isresponsible for the green luminescence, which isstronger under short-wave ultraviolet light. Whenthere is less than 1 ppm of U and less than 1000 ppm

of Fe, opals will luminesce blue (Figure 7, bottom).It has been shown that this blue luminescence is theresult of the nano-structured nature of silica in opal,offering abundant surface with dangling bonds. Thelast type of f luorescence in opal is the kind exhib-ited by pink opals. The rare pink opals from Mexico,Peru, and France all show a weak to moderate or-

Overview of Gem Opals: From the Geology to Color and Microstructure 15

Figure 7:Opal from centralMexico under incandescentlight (top) and sunlight(bottom). This opal pres-ents such a strong lumines-cence that it is visible undersunlight which is rich in ul-traviolet light. The green lu-minescence is due to theuranyl group

ange luminescence, which is more intense underlong-wave ultraviolet light. It is due to organic com-pounds called quinones, which are adsorbed onsome phyllosilicates contained in those opals (Gail-lou et al., 2012).

It was long believed that there was a differenceof luminescence between precious and commonopals. This study showed otherwise. In fact, it wasobserved that green luminescence was often exhib-ited by opals from a volcanic environment, whileblue luminescence was shown by opals formed in asedimentary environment. This is most likely due tothe availability of U in volcanic settings.

Micro- to Nano StructureThe archetypical image of opal structure is a perfectarrangement of silica spheres, about 250 nm in di-ameter (Figure 8). It is on this perfect network thatdiffraction can occur, giving rise to the desired play-of-colors. If every opal were to have this perfectstructure, they should all display play-of-colors,which we know not to be the case. What is thestructure of common opals, then? And do all pre-cious opals display this perfect arrangement? That iswhat we will develop here. The reader should referto Gaillou et al. (2008b) for more information.

The takeaway message is that only opal-A (theamorphous type) contains silica spheres. Opal-CThas a variety of micro-structures, which never comein the shape of a perfect sphere. We will describeeach variety of opal separately.

Opal-A

With play-of-color. A surprising fact is that there is nogem-quality opal known that has the perfect struc-ture of isolated spheres. The scanning electron mi-crograph (SEM) photo presented in Figure 8 is of achalky, very porous white opal from Tecopa in Cal-ifornia. In the gem variety, spheres are cemented to-gether by a (secondary) matrix of silica. The typicalstructure of a gem-quality opal with play-of-color ispresented in Figure 9a. There is not much to see in

16 Thirteenth Annual Sinkankas Symposium—Opal

Figure 9: SEM images of a play-of-color opal-A fromCoober Pedy, South Australia. A: On a fresh break, there isalmost no structure visible. B: After HF etching, the spher-ical structure is revealed.

Figure 8:A scanning electron microscope (SEM) image ofa play-of-color, chalky, porous white opal-A from Tecopain California. On a fresh break, it is extremely rare to seeindividualized silica spheres.

this picture, as cement and spheres are cindered to-gether to only reveal a homogeneous structure. It isonly after a brief etching process with hydrof luoricacid that the structure is revealed (Figure 9b). Thecement is in this case preferentially dissolved to ex-pose the spheres. For diffraction to occur, Bragg’slaw must be satisfied. Bragg’s law is defined by:

nλ = 2d sinθ

where n is an integer, λ is the wavelength of incidentwave, d is the spacing between the planes in theatomic lattice, and θ is the angle between the inci-dent ray and the scattering planes (Figure 10). Joneset al. (1964) noted that the silica spheres have to beapproximately 150–300 nm in diameter for diffrac-tion to occur.

Without play-of-color. The perfect arrangement of silicaspheres is not always achieved. Here are the differ-ent cases seen by Gaillou et al. (2008b):

1. Spheres do not have the same size (Figure 11).

2. Spheres do not have a perfect spherical shape(elongated spheres, for example).

3. Spheres have the same size but are not wellarranged.

4. Spheres are too small or too large to diffractlight.

Internal structure. Spheres of opal-A always show aninternal structure. Usually it is a concentric struc-ture, from barely visible up to a spectacular onion-shaped structure (Figure 11). The spheres are madeup of an arrangement of 25 nm nanograins (thebuilding block of the opal structure) organized inlayers. Rarely, spheres have a radial structure, aftera first episode of concentric growth (Figure 12), thatresembles a pineapple slice. This is typical of someopals from Honduras.

Overview of Gem Opals: From the Geology to Color and Microstructure 17

Figure 11:An SEM image of a common milky opal-A fromSlovakia, after HF etching. The spheres are very differentin size and present an onion-like structure.

Figure 10:The different variables of Bragg’s law. Source: Wikipedia, free of copyright.

Opal-CTWith play-of-colors. Play-of-color opal-CT looks muchdifferent from opal-A. Its structure is based on per-fectly arranged “lepispheres” (Figure 13a). The lepi-spheres contain an arrangement of silica platelets(blade-shaped) that gives rise to a globally sphericalshape, comparable to a desert rose. Platelets aremade of nanograins approximately 25 nm in diame-ter. As with opal-A, the lepispheres of opal-CT arerarely seen on a fresh break, as most of the timethere is a cement of nanograins between thespheres. After hydrof luoric acid etching, the lepi-spheres are dissolved, leaving only the cement be-tween the lepispheres (Figure 13b).

Without play-of-colors. Common opal-CT has a varietyof structures. When common opal-CT is composedof lepispheres, the lepispheres either (1) have differ-

18 Thirteenth Annual Sinkankas Symposium—Opal

Figure 13:SEM images of play-of-color opal-CT. A: Thisopal from the El Cobano mine in Jalisco state, Mexico, ischalky white and shows a lepispheric structure on a freshbreak. B: This chocolate opal from Mezezo, Ethiopia, onlyreveals the casts of the lepispheres after an HF etch.

Figure 14:SEM images of common opal-CT. A: This whiteopal from the Las Crucitas mine in Jalisco state, Mexico,shows a tablet structure. B: This fire opal from theOlimpia mine in Querétaro state, Mexico, has a structureof disordered nanograins.

Figure 12:An SEM image of a colorless opal-A from Hon-duras, after HF etching. The spheres have an unusualpineapple-like structure, with concentric and then radialgrowth.

ent sizes, (2) have the same size but are not wellarranged, or (3) are too small or too large to diffractlight. But the lepispheric structure is not as commonas other structures, such as platelets. Platelets canbe stacked together, giving rise to intergrown tablets(Figure 14a). The most typical scenario, especiallywhen it comes to common fire opal, is a structureof disordered 25 nm nanograins (Figure 14b). Finally,in the case of pink opals that contain a phyllosili-cate, a fibrous structure is observed, as the phyllosil-icate fibers serve as templates for the nanograins.

ConclusionThis article gives a general overview of gem opals,from its formation to its body colors, play-of-colors,and internal secrets. We showed that gem opal isgenerally formed by circulating f luids in silica-rich

rocks. The body color is always due to nano- ormicro-inclusions. The luminescence is related toimpurities as well as intrinsic defaults in silica. Thestructure of gem opal-A and opal-CT are formed bynanograins about 25 nm in diameter that couldarrange to give rise to many structures. Gaillou et al.(2008a) showed that there was no correlation be-tween the presence of some chemical elements andopal type (opal-A versus opal-CT), or the differentstructures observed. There must be other factorsthat control the precipitation of silica in the varietyof structures seen, such as the stability of the envi-ronment, the temperature, the pH, the silica satura-tion, or the evaporation rate. Those have yet to beidentified. The best way would be to replicate thevariety of structures by reproducing them in the lab-oratory. So far, only opal-A and opal-CT with lepi-spheres have been synthesized.

Overview of Gem Opals: From the Geology to Color and Microstructure 19

Bayliss, P., and Males, P.A. 1965. The mineralogical similarityof precious and common opal from Australia. MineralogicalMagazine 35(270): 429–431.

Fritsch, E., Rondeau, B., Ostrooumov, M., Lasnier, B., Marie,A.-M., Barreau, A., Wery, J., Connoué, J., and Lefrant, S. 1999.Découvertes récentes sur l’opale. Revue de Gemmologie a.f.g.138/139: 34–40.

Gaillou, E., Delaunay, A., Rondeau, B., Bouhnik-Le Coz, M.,Fritsch, E., Cornen, G., and Monnier, C. 2008. The geochem-istry of gem opals as evidence of their origin. Ore Geology Re-views 34(1–2): 113–126.

Gaillou, E., Fritsch, E., and Massuyeau, F. 2012. Luminescenceof gem opals: a review of intrinsic and extrinsic emission.The Australian Gemmologist 24(8): 200–201.

Ilieva, A., Mihailova, B., Tsintsov, Z., and Petrov, O. 2007.Structural state of microcrystalline opals: A Raman spectro-

scopic study. American Mineralogist 92(8–9): 1325–1333.

Jones, J.B., Sanders, J.V., and Segnit, E.R. 1964. Structure ofopal. Nature 204(4962): 990–991.

Rondeau, B., Fritsch, E., Mazzero, F., Gauthier, J.-P., Cenki-Tok, B., Bekele, E., and Gaillou, E. 2010. Play-of-color opalsfrom Wegel Tena, Wollo Province, Ethiopia. Gems & Gemology46(2): 90–105.

Rondeau, B., Gauthier, J.-P., Mazzero, F., Fritsch, E., Bodeur,Y., and Chauviré, B. 2013. On the origin of digit patterns ingem opal. Gems & Gemology 49(3): 138–146.

Sosnowska, I., Buchenau, U., Reichenauer, G., Graetsch, H.,Ibel, K., and Frick B. 1997. Structure and dynamics of the opalsilica-water system. Physica B: Condensed Matter 234–236: 455–457.

Webster, R. 1975. Gems: Their Sources, Description and Identification,revised by B.W. Anderson, 3rd ed. London, Butterworths,199–209.

References

AcknowledgmentsI want to give special thanks to Stuart Overlin, who reviewed this article. This work has been conducted mainlyduring my PhD, under the supervision of Prof. Emmanuel Fritsch, who I would like to acknowledge here.

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